Appearance Error

For the second time in 3 months, each time Sydney updates, I lose control of my Appearance Editor. I receive a lengthy error message.
What’s going on?
I’ve been using this theme without issue for a few yrs yet I am ready to change.
I’ve done the usual deactivating all plugins for compatability issues and that doesn’t remedy the problem.
Can the creator of Sydney please address this or you will lose me soon.

What does the error message read? Is it also appearing with the default WordPress theme like Twenty Seventeen?